10 Reasons Lie Detectors Are Completely Unreliable

The concept of "lie detection" is nothing new. The Bedouins used to force witnesses to lick hot iron, thinking that "lying causes a dryness of the mouth" and only liars would have burned tongues. Modern detectors (aka polygraph machines) are slightly more sophisticated, but are lie detectors accurate? What is the science of lie detectors, anyway?

Despite their ubiquity in film and television, and the FBI's insistence on using them as a screening tool, the scientific community agrees there are many reasons lie detectors don't work. As one critic noted, it's "19th-century technology... put in a box." While they may have some value as a prop or deterrent, experts think their continued use is "highly problematic." Why? Read on and don't worry about your heart rate - it doesn't tell us anything.

  • Deception Doesn't Manifest Itself Physically

    Deception Doesn't Manifest Itself Physically
    Photo: Parks & Recreation, 2013 / NBC

    Unlike the way, say, being afraid can cause your body to react in certain ways, the act of deception doesn't cause your body to exhibit unique physiological signs indicating that you are, in fact, a liar. A polygraph measures heart rate, blood pressure, sweating, and breathing, none of which are linked in any way to lying or truth-telling, according to psychologist Leonard Saxe. (Saxe should know: he authored a 1983 report to Congress that led to a nationwide ban on private employers forcing employees to take polygraph tests.)

    The basic theory behind modern polygraph testing - lying manifests itself in your body - is similar to an ancient Chinese trick for interrogating suspected criminals. The interrogator would fill the suspect's mouth with uncooked rice, because liars, the logic went, have dry mouths, meaning if rice stuck to your tongue, you were considered a liar. Polygraphs rely on a similar misconception, made only slightly more sophisticated via a gauntlet of pre-test control questions meant to calibrate what your lie looks like (i.e., no, polygraphy advocates "do not claim that the instrument measures deception directly").

  • Operators Get to Decide What a Lie Looks Like

    Let's not forget: behind every lie detector is a lie detectorist running the show. The person conducting the polygraph test asks "control questions" that establish what your version of "lying" looks like. What if they're wrong?

    Mother Jones reports that a man named George Maschke found out the hard way how an operator, or polygrapher, can ruin test results. Maschke applied to be an FBI special agent but failed because of "deception with regard to each and every relevant question." Since some of those "relevant" questions involved selling narcotics and treasonous acts, Maschke was shocked.

    He chalks the failure up to a control question about driving under the influence, something Maschke says he has honestly never done. The polygrapher, Maschke says, assumed that everyone who drinks has driven when they shouldn't have, and "marked" his answer as an example of a lie, screwing up the rest of the results. Maschke went on to create AntiPolygraph.org.

  • Polygraph Tests Really Only Measure Anxiety

    Polygraph Tests Really Only Measure Anxiety
    Photo: Snowden, 2016 / Open Road Films

    If there was ever a need for a machine to measure anxiety, we wouldn't have to invent it: polygraph machines are quite good at it already. The trouble is, anxiety isn't necessarily associated with veracity.

    In fact, frequent liars exhibit less anxious behavior simply because they're better at it. Two convicted federal agents, Robert Hanssen and Aldrich Ames, had no problem breezing through polygraph tests during their long careers. It helps, of course, that they were both double agents spying for the Soviet Union and Russia, meaning they both lied for a living.

  • Being Falsely Accused Is Arousing, Too

    Being Falsely Accused Is Arousing, Too
    Photo: Meet the Parents, 2000 / Universal Pictures

    Another way of looking at lie detectors is as arousal detectors. Anxiety, fear, deception, and the like are all fairly subjective and hard to measure, but sheer arousal is easily quantified via polygraph.

    However, being falsely accused can manifest a similar physiological arousal as being caught dead-to-rights can, meaning lie detectors are actually biased toward innocent individuals, according to Dr. William Iacono, writing in the Journal of Forensic Psychology Practice. Furthermore, the pre-test control questions, Iacono argues, simply can't evoke an emotional impact anywhere near as substantial as being hit with a "relevant" question, largely because most subjects assume the control questions are "trivial." This seriously undermines the basic premise of lie detection.

  • They Are Beatable with Training

    They Are Beatable with Training
    Photo: Penn & Teller: Bullsh*t!, 2009 / Showtime

    It doesn't take an international super-spy to beat a polygraph test. LAPD recruits, for example, reportedly swap "tip sheets" detailing methods to beat the test, even right outside the exam room. Experts agree they're beatable, noting all of the necessary physiological signs "can be altered by conscious efforts through cognitive or physical means." The federal government has even targeted instructors offering lessons in polygraph "countermeasures."

    Perhaps the most shocking example is the failed test of Gary Ridgway, a man later found to be a serial killer, thanks to DNA evidence. He claimed he beat the test by "just relaxing."

  • The Placebo Effect Muddies the Waters

    The Placebo Effect Muddies the Waters
    Photo: Basic Instinct, 1992 / Tristar Pictures

    A lie detector can't determine if a subject is influenced by the placebo-like effects manifested when they're hooked up to a strange machine and grilled about their past, which is a definite problem. If a subject believes, strongly, that the test works, they may be way more anxious than a skeptic would be. This naturally leads to issues both during the control question phase and the actual interrogation. This concern is why the American Psychological Association says the lie detector "might be better called a fear detector."